1. Field of the Invention
The subject invention generally pertains to electronic power conversion circuits, and more specifically to high frequency, switched mode power electronic converter circuits.
2. Description of Related Art
The most common device used for high speed switching in high frequency power converters is the power mosfet. For some high power applications bi-polar devices or combination MOS/bipolar devices are used, but these are used at lower switching frequencies due stored charge effects. To achieve the highest power densities mosfets are required because achieving high power densities requires operation at high switching frequencies. Also, at high voltages a soft switching mechanism is often used to eliminate first order switching losses. The zero voltage switching (ZVS) phase shifted full bridge forward converter is a common choice for high power off line applications.
The power mosfet is the best choice for high frequency, high power, and high density applications,:but it is not a perfect switch. The on resistance of the switch is higher than bi-polar devices so that conduction losses may be the dominant loss mechanism, for ZVS applications, and, although the mosfet switch is fast there are still some switching losses. In order to achieve higher efficiencies one can parallel power mosfets or use very large dies which, in both cases, amounts to using more silicon. Using more silicon reduces conduction losses in the switch but also increases switching losses. The switching losses are increased with larger silicon because gate capacitance increases with large silicon and the increased gate capacitance results in both higher gate drive energy and slower switching. The slower switching does not matter so much for the ZVS turn on transition except that the timing of the turn on transition is often critical and faster switching makes the turn on transition timing less critical, but switching losses during the turn off transition depend on switching time. During the turn off transition as the channel turns off the current shifts from the channel of the mosfet to the parallel capacitance. Current in the channel creates losses and current in the parallel capacitance is lossless so that the faster the current in the channel can be transferred to the parallel capacitance the lower the switching losses will be.
An object of the subject invention is to provide a composite switch with lower conduction losses than a single mosfet switch or parallel combination of mosfet switches.
Another object of the subject invention is to provide a composite switch with reduced gate drive losses.
Another object of the subject invention is to provide a composite switch with reduced control input capacitance and faster switching.
Further objects and advantages of my invention will become apparent from a consideration of the drawings and ensuing description.
These and other objects of the invention are provided by a novel circuit technique that uses two mosfets arranged in cascode and two capacitors. One of the mosfets is a low voltage device with a small die size and low on resistance, the other mosfet is a higher voltage device with higher on resistance and a larger die. One of the capacitors provides a fixed voltage for driving the high voltage mosfet. The second capacitor is used to limit the voltage applied to the smaller low voltage mosfet.